An ad that ran briefly on MSNBC last week highlighting John McCain's cancer scars -- complete with unflattering images of the GOP nominee's bandaged face after his melanoma surgery eight years ago -- was pulled from the air after Fox News commentator Bill O'Reilly lambasted the spot, calling it "the most vicious political ad of the campaign."

"McCain's Medical Records" (subscription), released by left-leaning groups Democracy For America and Brave New PAC, began airing on MSNBC Thursday. It pairs two doctors' grim diagnoses of melanoma patients with close-up images of McCain's cancer scars and urges McCain to release his medical records in a more thorough fashion than he did in the spring.

NBC spokeswoman Allison Gollust told the Associated Press that "the ad had not been thoroughly vetted prior to air, and has since been removed from our commercial rotation."

"I have no idea what that means," said Democracy for America communications director Daniel Medress. "Both of the gentlemen in the ads are doctors. John McCain is 72 years old. He's had cancer four times. What wasn't vetted?"

CNN also refused to air the ad, without citing a specific reason, Medress said.

MSNBC pulled the ad a day after O'Reilly denounced it on his cable show. Conservative talk radio host Rush Limbaugh also reacted angrily to the spot.

The ad is getting some play in the world of progressive Internet media, however, including the liberal talk show "The Young Turks," and Medress hopes the mainstream media will soon follow suit. "The first part of this is having our members reach out to NBC; hopefully they’ll come to their senses on this thing," Medress said. "This is becoming a matter of getting out there into both the traditional press and also progressive online media."

So what do the doctors featured in the ad have to say? Michael Fratkin told his local paper, The Eureka (Calif.) Times-Standard, that he felt compelled to appear in the spot partly by McCain's vice presidential selection: "The idea of Sarah Palin as president of the United States fills me with grave concern, all due respect to her."

This isn't the first go-round this year for Brave New PAC and Democracy For America. They released another ad following the GOP convention that showed a fellow POW casting doubt on McCain's fitness for office.

While the groups had intended to start running another attack ad this week, in light of the controversy surrounding their latest ad, they're holding off on releasing it. The topic of that spot? McCain's "dishonesty." Medress said: "It will be legitimate. But will it be hard-hitting? You betcha."